Sensing Viscosity Decoupling and Dynamic Heterogeneity in Tetrabutylammonium Bromide:Polyol Deep Eutectic Solvents

We report the reorientation dynamics of the chromophores resorufin and perylene in tetrabutylammonium bromide (TBAB)-based deep eutectic solvent (DES) systems formed with ethylene glycol (EG), 1,3-propanediol (PD), and 1,4-butanediol (BD) across a series of constituent ratios for each DES system. The reorientation dynamics of both chromophores are largely independent of the bulk viscosity of the DESs, consistent with viscosity decoupling and dynamic heterogeneity. This behavior suggests that the characteristic domains sampled by the chromophores are larger than their hydrodynamic volumes. For the polar chromophore resorufin, the results provide evidence for relatively strong interactions with TBA+ in the TBAB:PD and TBAB:BD systems, but not in TBAB:EG, suggesting competitive interactions between hydrogen-bond-acceptor-rich domains and hydrogen-bond-donor-rich domains. TBAB-based DESs, therefore, differ from choline chloride-based DESs primarily because TBAB lacks the primary alcohol functionality present in choline chloride.